1. Field of the Invention
The present invention relates to a digital data radio transmission system.
2. Description of the Prior Art
Digital data radio transmission systems involve having the transmitting side subject input frame data to velocity modulation and pulse insertion in order to transmit to the receiving side such signals as frame synchronizing, auxiliary and control signals. Generally, no compatibility exists between the format of input frame data and that of radio frame data after velocity modulation. This makes it impossible for a repeater station to reproduce the input frame data from the radio frame data. The same applies to a digital data radio transmission system that is built in accordance with the so-called network node interface (NNI) protocol.
A transmitter of a typical prior art digital data radio transmission system is described below with reference to FIG. 1. This transmitter subjects eight-bit parallel input data to velocity modulation and pulse insertion so as to generate eight-bit parallel output data for radio transmission to a repeater station.
In FIG. 1, reference numeral 1 denotes a sextet write counter. The write counter 1 divides input write clock pulses WCK by six (6). The divided pulses are output as write pulses WP to an elastic memory 2. The elastic memory 2 is an eight (8) bit by six (6) bit memory that stores input frame data IFD whose format is defined by the NNI, as shown in FIG. 2A. The elastic memory 2 admits and stores the input frame data IFD as per the write pulses WP, and outputs the data in accordance with read pulses RP. The output data is data that has undergone velocity modulation.
The input frame data IFD depicted in FIG. 2A is eight-bit parallel data, one frame of which contains 2,430 bits. Each frame is assigned a frequency of 8 kHz. In addition, one frame comprises nine (9) sub-frames, i.e., 1 row through 9 row, each sub-frame being made of a nine-bit overhead part O shaded in the figure and a 261-bit payload part P. Maintenance-related signals are inserted into the overhead part P while communication information is inserted into the payload part P.
Referring again to FIG. 1, the write pulses RP destined to the elastic memory 2 are read from a read counter 3 comprising a sextet counter. The read counter 3 admits read clock pulses RCK and clock inhibit control pulses IP, and outputs the write pulses RP. The clock inhibit control pulses IP are generated by a timing pulse generator 4 that admits the read clock pulses RCK.
When the data in the elastic memory 2 is read therefrom in accordance with the read clock pulses RCK and is output as output frame data OFD, the clock inhibit control pulses IP are used to place blank bits in the date OFD. That is, as depicted in FIG. 2B showing the format of radio frame data RFD, each 270-bit sub-frame (#1-#8) is followed by a one-bit slot ST.
A pulse inserting section 5 shown in FIG. 1 inserts an auxiliary signal S in each slot ST. These slots and sub-frames constitue radio frame data RFD that is transmitted to a repeater station, not shown.
In the above-described prior art digital data radio transmission system, the input frame data IFD depicted in FIG. 2A is subjected to velocity modulation and then to pulse insertion so as to form the radio frame data RFD of FIG. 2B. Because the input frame data IFD is out of frame phase with the radio frame data RFD and because the radio frame data RFD after velocity modulation is slightly narrower in bit width than the input frame data IFD, the two kinds of data differ from each other in terms of frame period. The mismatch in frame period between input frame data and radio frame data makes it impossible to reproduce the IFD from the RFD.
More specifically, when a repeater station receives radio frame data RFD, the station cannot gain access to the overhead part 0 of input frame date IFD in the NNI-defined initial format and is hence incapable of reproducing the IFD. This disadvantage leads to further disadvantages including the inability of the repeater station to add, delete or modify the data it handles. Such problems have conventionally hampered the system operation considerably.